This invention concerns a method for determination of the location of terminal equipment in a cellular mobile communications system including several base transceiver stations and wherein the terminal equipment may move from one cell to another and from one traffic area to another during the traffic connection.
When a terminal in a mobile telephone network is in a traffic or signalling connection with the network, information about the location and movement of the terminal equipment in the network has not been of primary importance in most cases. However, in some cases information about the location and movement of the terminal equipment is a desirable network operator service. One such case is e.g. when the authorities for some reason wish to trace the movement of a certain terminal in the network.
The following is a description of the structure and operation of a mobile telephone network using as an example the GSM mobile telephone network shown in FIG. 1. Other digital mobile telephone networks include the same network elements as those in the GSM network. Communication between the network and mobile station MS in the cell takes place via the radio path by way of base transceiver station BTS. Base transceiver stations BTS are connected to base station controller BSC. Several base transceiver stations BTS are usually under the control of one base station controller BSC and they can be chained to one another or cabled directly to the base station controller, as is illustrated in the figure. The geographical area covered by these base transceiver stations is called location area LA and the calls intended for the mobile stations in this area are sent through these base transceiver stations. FIG. 1 shows six location areas. The mobile station may move within the location area without any need to update the location data concerning the mobile station.
Several base station controllers are connected to one mobile switching centre MSC, which carries out the main switching functions of the mobile telephone network. In addition, it connects the mobile telephone network with external networks. The entity of location areas under the management of the mobile switching centre is called the switching centre area. In the figure, the switching centre area of the MSC 1 switching centre includes location areas a, b and c, while the switching centre area of the MSC 2 switching centre includes location areas d, e and f. All calls beginning from and ending in the switching centre area are relayed by way of the same mobile switching centre. If a mobile station network includes several mobile switching centres like switching centres MSC 1 and MSC 2 in the figure, their switching centre areas together form the system area of the mobile communications network, which means the geographical area covered by one network. The network is almost always run by one operator.
The mobile telephone network also includes various databases. Subscriber data is stored permanently in home location register HLR, irrespective of the current location of the subscriber. If the network is large and there are many subscribers, then several home location registers are used, in the figure these are HLR a, HLR b and HLR c respectively. The number space is divided to these registers, whereby the search for subscriber data is quicker. Visitor location register VLR is nowadays integrated with each mobile switching centre and the subscriber data fetched from the home location register is stored therein while the subscriber visits the area of the VLR, that is, the switching centre area. For example, in FIG. 1 subscriber data of a mobile station located in cell 1 is stored in home location register 5 connected to mobile switching centre MSC 1. Mobile station MS may move freely within the location area a without any need to update this data.
Location update is done every time when the subscriber connects to the network. When a subscriber e.g. in cell 1, FIG. 1, turns on his mobile station MS, it signals a location update request to base transceiver station 2, and the request is directed by way of base station controller 3 to mobile switching centre MSC 1 and further to visitor location register VLR indicated by reference number 5. The register asks the subscriber to send his subscriber code, from which the VLR learns the subscriber""s home location register, which in this case is HLR a. Visitor location register VLR asks home location register HLR a to send the authentication parameters needed for the identification of the subscriber and for encrypting the connection. After a successful authentication of the subscriber, home location register HLR a knows that area of the visitor location register, that is, the mobile switching centre, in which the subscriber is located and it sends subscriber data to visitor location register 5 in question. The subscriber""s location is now updated, that is, HLR knows the address of VLR 5, and VLR 5 knows the location area a, wherein the mobile station is located.
Location update is also done every time when a subscriber moves from one location area to another, e.g. when in FIG. 1 mobile station MS moves from location area 11 to location area 12. This update is done on the mobile station""s initiative, which informs the visitor location register about the changed location area. If the location area belongs to the same switching centre area, that is, to the control of same visitor location register VLR, as in our example, then there is no need to change the address of visitor location register 5 which is stored in home location register HLR a. The address is changed only if the subscriber moves to another switching centre area, e.g. from location area c to location area d, whereby visitor location register 7 tells its address to home location register HLR a, and the home location register sends the subscriber data to visitor location register 7.
Location update can also be made periodically at certain intervals (Periodic Location Update). Hereby the location update is triggered off when the time counter in the mobile station is full.
Based on the above it is known that the home location register always knows the visitor location register connected to the mobile switching centre in whose mobile services switching centre area the subscriber is located, and the visitor location register knows in which location area the subscriber is located. The code of the location area is unambiguous. When the subscriber is moving in any network, this data is updated. This is also true in part, if the subscriber is moving in the network of such another operator, with whom the own operator has a roaming contract. In this case the only thing which the home location register knows about the subscriber is whose network the subscriber is registered with. In case of a foreign country, what is known is in which country and in which operator""s network the subscriber is located.
System management is an important part of the mobile telephone system. In most cases the management function is centralised in an Operation and Maintenance Centre OMC, from which the configuration, management and testing of network elements and downloading of software etc. are performed by remote control and in a centralised manner. In the figure, the dashed lines beginning from operation and maintenance centre 8 and ending in network elements illustrate management connections. Another important mobile telephone system is the network planning system, which is used for computing the network topology when new cells are added, old ones are removed and cell sizes are changed. This network planning system knows the co-ordinates of base transceiver stations and the coverage areas of cells. Network planning can also be included in the system management.
Reference is now made to FIG. 2 and to FIGS. 3A-3B. FIG. 2 shows in a condensed manner that information essential for the invention which the different network elements have about the subscriber and which information is stored on the subscriber card of terminal equipment MS. The figure corresponds to FIG. 1 in that the area in question is the mobile services switching centre area of switching centre MSC 1, wherein there are three location areas. Each location area has its own location area identity LAI, which are here marked with letters a, b and c. The contents of the location area identity is shown in FIG. 3B. The identity consists of mobile network country code MCC, in Finland the code is 244, mobile network code MNC, which is an operator-specific parameter, and location area code LAC. The operator""s location area identities thus differ from each other only as regards the LAC code. In Finland, the number of granted mobile network codes is 10 and e.g. the operator called Telecom Finland uses the MNC codes 91 and 92.
Location area identity LAI is in the memory of base station controller BSC of this area. When a mobile station is in some location area it has also received from the network this location area identity LAI and has stored it in its memory. Mobile subscriber international ISDN number MSISDN(telephone directory number) and International Mobile Subscriber Identity IMSI are stored on the mobile station""s SIM card. When the mobile station has registered with the network, its location area identity LAI, its MSISDN telephone number and its IMSI code are stored in visitor location register VLR of switching centre MSC. It is not necessary to store the MSISDN number on the SIM card, but in connection with the location update the number is transferred from the home location register to the visitor location register together with the other subscriber parameters.
The visitor location register also contains a pool of so-called mobile station roaming numbers MSRN. This number is used when a call is coming to the mobile station. Hereby VLR""s reply to HLR""s enquiry about the subscriber""s location is to take one MSRN from the pool, to give it temporarily as the subscriber""s number and to send this number to the enquiring HLR. Based on the MSRN number the call can be routed through transit networks to this switching centre and, according to the characters of the final part of the subscriber number, further to the subscriber. Upon completion of the call the mobile station roaming number is released for reuse.
The contents of the MSISDN connection number are as shown in FIG. 3A. It contains country code CC, e.g. if a Swedish subscriber is in Finland, the country code is +358, national destination code NDC, which is the operator""s routing code in practice and which in Finland is 40 in the network of the operator Telecom Finland, and the xe2x80x9csubscriber numberxe2x80x9d. The last number is subscriber number SN individualising the subscriber.
Based on the description in accordance with FIGS. 2 and 3A-3B, home location register HLR always stores the subscriber""s connection number MSISDN, subscriber""s international mobile subscriber identity IMSI and the address of that visitor location register VLR where the subscriber is located. Besides the subscriber""s MSISDN and IMSI numbers, the visitor location register also stores the LAI identity and thus the LAC code of that location area where the subscriber happens to be at that moment. When MS is moving from location area a to location area b in FIG. 2, the LAC code is the only code that will change in the memories of the mobile telephone and the base station controller. No changes take place in the home location register.
Known mobile telephone systems offer a possibility to locate the terminal equipment. The location accuracy may be one cell. Hereby, when a mobile station starts a call or when a call is set up to a mobile station, the base transceiver station will from the call set-up message signalled by the mobile station learn the identity of the cell where the mobile telephone is camping. The mobile station for its part has received the identity from the broadcasting channel transmitted by the base transceiver station. The identity in the message may function as a triggering off to the intelligent network.
The location accuracy may also be less than one cell. Hereby the system must be such that also neighbour base transceiver stations will receive the mobile station""s transmission, whereby the location can be figured out from the time differences between transmissions.
It is also possible to start such a programme in a mobile switching centre, which stores e.g. tracing data obtained during the call set-up in the disk memory, from which they can be read afterwards. This method will load the switching centre""s memory very much, so there may not be very many pieces of terminal equipment to trace.
Drawbacks of the known location determination methods are that the location can be determined only when the mobile station is setting up a call and during the call. This means that the tracing data is deficient. When using an intelligent network it is difficult to route a call to the intelligent network""s service switching point SSP.
The objective of this invention is a terminal tracing system which does not suffer from the drawbacks of known methods. It is an objective to be able to trace movement of the terminal in the network irrespective of whether it is in a traffic connection or in an idle state just listening to paging calls. It is a particular objective to utilise as much as possible the already existing features of the mobile telephone network.
The established objectives are achieved with the definitions presented in the independent claims.
The invention utilises that feature of mobile telephone systems that the location of a terminal with location area precision is always known to the system and that the geographical network topology is known to the network planning system.
The tracing system includes three functional parts: the first part gives to the mobile telephone system as input enquiry the connection number of the terminal equipment to be traced and receives, as a response to the input enquiry, information on where to ask for the subscriber""s location area. The second functional part enquires about the subscriber""s location area and receives as a response the location area code which unambiguously identifies the location area. The third functional part fetches information corresponding to the subscriber""s location area from the location area table and attends to reporting on the results.
Based on cell topology information obtained from the network planning system, a location area table has been set up in advance, which converts the location area identity into an area name which is in plain language and is bound to the map. The location area table preferably contains the following for each location area:
the location area identity in the form in which it is available from the network,
an equivalent geographical location area computed from the real geographical location area which includes coverage of the cells of the area and the co-ordinates of the base transceiver stations of the cells,
co-ordinates of the centre of gravity of the location area,
a given suitable geographical name which is chosen by placing an equivalent location area on top of the geographical map containing the place names.
When the network gives the location area identity of the subscriber station as a response to the connection number, the tracing system will convert the identity into a geographical place name. Finally, the tracing system creates a record containing at least the subscriber connection number and the geographical place name where the subscriber is located.
Enquiries can be made at desired intervals and the result can be stored in a tracing file. From this a report can be formed, from which it is easy to find out the subscriber""s movement with location area precision.
The report may be supplemented by adding such other desired information to it which the mobile telephone system produces naturally. Such information is at least the time of day and the date, whereby the terminal has done the location update.